Passengers and crew of aircraft and seafaring vessels must be provided with life jackets that have Water Activated Batteries (WABs) powering survivor locator lights. WABs for use on ships typically include lithium based power cells which are safe, reliable and economical. However, due to the low probability for explosions, lithium based power cells are forbidden to be used in WABs for aircraft. Instead, WABs have been developed which contain dry electrodes connected to a lamp, such that immersion in water causes the water to enter apertures in the outer casing of the WAB, fill the gap between the electrodes and dissolve a salt contained in the WAB to provide ions so that an ionic current is created between the electrodes powering the lamp.
According to Aerospace Standard AS4492 for Survivor Location Lights, Water Activated Batteries are required to light the lamp with a specified light intensity within ten minutes of immersion in fresh water and within 5 minutes of immersion in seawater, and to maintain this light intensity for at least eight hours. Such Water Activated Batteries are also required to have a long shelf life. Unfortunately humidity tends to cause the same reactions as those that immersion activates, and some parts of the world, such as Jamaica, Florida and Hong Kong, have hot humid environments. The chemicals used in water activated batteries thus need to meet the conflicting requirements of being stable in hot, humid environments, yet easily activated by immersion in water.
Deferred-action batteries of this type have been known for decades, and various embodiments of such batteries and methods for the production and use thereof, as well as for the manufacture of their component parts, have been described, inter alia, in U.S. Pat. Nos. 2,491,640; 2,636,060; 2,655,551; 2,658,935; 2,716,671; 2,817,697; 3,343,988; 3,859,136; 3,953,238; 4,016,339; 4,192,913; 4,261,853; 4,332,864; 4,368,167; 4,487,821; 4,803,135; and 4,822,698.
U.S. Pat. No. 2,491,640 describes a voltaic cell of this type that is intended especially for use in operating an emergency signal at sea. The signal may be a light to indicate the presence of a person who has become stranded by shipwreck or other causes. It may also be an electronic apparatus floating on the water that emits a signal which can be detected at a distant point. The battery is adapted to power the signal and to be activated by immersion in water, which may be the fresh water of an inland lake or river, or the salt water of the ocean.
Such batteries essentially comprise an anode which is usually a magnesium alloy, and a cathode that has traditionally been a silver or copper halide, wherein discharge of the stored energy is initiated by immersing the battery in seawater, which serves as a conducting electrolyte between the anode and the cathode.
Most of the earliest patents that relate to voltaic cells of this type use cathodes based on cuprous chloride, while more recent patents such as U.S. Pat. Nos. 4,192,913 and 4,261,853 describe cathodes based on cuprous thiocyanate.
For example, U.S. Pat. No. 5,424,147 to Khasin et al, describes a water-activated, deferred-action battery having a housing containing at least one cell, comprising at least one anode selected from the group consisting of magnesium, aluminum, zinc and alloys thereof; a cathode comprising a skeletal frame including conductive metal and having a portion of its surface area formed as open spaces, and further comprising a heat-pressed, rigid static bed of active cathode material encompassing the skeletal frame, the cathode material being formed of cuprous chloride, sulfur, carbon and a water-ionizable salt and being compacted and fused under pressure and heat to itself and to the skeletal frame, to form a heat-fused, conductive, electrochemically active phase; at least one cavity separating the cathode and the at least one anode, and at least one aperture leading to the at least one cavity for the ingress of an electrolyte-forming, aqueous liquid.
Other cathodes described in the literature include copper sulfate, lead chloride, copper iodide, lead oxide or potassium persulfate as the active material. However, all of the above materials suffer from one or more of the following problems:                Sensitivity to humidity upon storage resulting in expansion of the cathode until the cavity between the anode and cathode no longer exists        Shorting the electrodes and discharging the battery, fast dissolution in water upon activation which results in early failure        High cost        Use of toxic materials.        
Various methods are well known to protect cathodes from high humidity. These include water soluble films that have a low water vapor transmission rate (WVTR), and mechanical plugs that block the apertures that are automatically removed by inflating the lifejacket. Unfortunately such water soluble films only have a low WVTR in low or medium humidity, but in high humidity the films have a high WVTR and so are not effective. In addition after prolonged storage of many years it may take many minutes for them to dissolve. Automatic removal of plugs is an expensive and bulky addition to each lifejacket.
While remaining fully compliant with International Aerospace Standards, there is an interest in prolonging the product life of survivor locator lights in high humidity environments, in shortening the period for the light to come on following immersion, and in reducing unit cost.